1. Field of the Invention
This invention relates to a sheet stacking unit disposed on a printed sheet tray of a printing machine, and more particularly to a unit for stacking small-size printed sheets such as postcards.
2. Description of the Prior Art
Stencil duplicating machines are well-known, in which an optically read original image is digitally perforated on a stencil, and the stencil is wrapped over a printing drum so that the image is printed on sheets via the stencil.
Such stencil duplicating machines are prone to a problem that when a currently printed sheet is placed on a previously printed sheet on a printed sheet tray, a rear surface of the currently printed sheet may be smeared with ink from the previously printed sheet. This phenomenon is called "back transfer". Especially, front and rear surfaces of postcards are required to be free from any smears except for the messages printed thereon. It is therefore difficult to print the postcards without back transfer. To overcome this problem, a variety of methods have been known up to now.
The following three methods have been proposed.
1. Each postcard is printed by operating a trial print key after a previously printed postcard is removed from a printed sheet tray:
2. Each postcard is printed at a slowest speed, and a slip sheet is placed on a previously printed postcard in a printed sheet tray: and
3. Each postcard is printed by each fifth rotation of a printing drum, a sheet feeding unit feeds each postcard in synchronization with the printing drum, and each printed postcard is removed from a sheet tray so as to be placed on a table or the like.
These three methods are somewhat disadvantageous because they are cumbersome and considerably time consuming.
Further, Japanese Patent Laid-Open Publication No. Hei 5-016,514 proposes a method in which printed postcards are sequentially placed on a rather long sheet feeding route between a printing drum and a printed sheet tray, so that ink penetrates into the postcards. Japanese Utility Model Laid-Open Publication No. Hei 5-013,743 exemplifies a method in which each printed sheet is carried from a printing machine to a sheet tray at a predetermined interval within a predetermined period of time.
However, a larger printing machine is necessary to provide a longer sheet feed path. If printed sheets are jammed in the sheet feed path, they cannot be removed easily. When printed sheets are carried to a printed sheet tray with a predetermined space kept between adjacent sheets, a printing machine becomes inevitably large and expensive. In this case, when a height of a sheet discharging outlet of the printing machine is taken into consideration, a sheet stacking unit should be of a floor type. This means that it is rather difficult to remove printed sheets. Further, when printing thin sheets or sheets of an ordinary size, the sheet stacking unit should be detached from the printing machine and a printed sheet tray should be attached. This detachment and attachment of the sheet stacking unit and the printed sheet tray is inconvenient.
The sheet stacking units disclosed in the foregoing Japanese laid-open publications do not include members for notifying an operator or a printing machine of any problem such as jamming or a full state of a sheet receptacle of the sheet stacking unit. Even when printed sheets are not reliably carried in the sheet stacking unit, the printing machine continues feeding subsequently printed sheets, which would gather at the sheet stacking unit and are jammed there. This means that the efficiency of the printing machine will be reduced. Further, if jamming is not detected promptly, the sheet stacking unit may be damaged. When the sheet receptacle is full, printed sheet may overflow therefrom or may be packed therein. In the latter case, back transfer may be caused on the printed sheets.
When the printing machine and the sheet stacking unit are electrically connected, at least one of them should be mechanically and electrically modified for such a connection. Such a connection is difficult to accomplish.